Every plant, animal or other nucleus-containing cell also harbors an array of miniature "organs" that perform essential functions for the cell. In plants, for example, organelles called chloroplasts photosynthesize to generate energy for the organism. Because some organelles contain their own DNA and resemble single-celled organisms, scientists have long theorized that the evolution of complex life forms got its start when one cell engulfed another and the two learned to live in harmony—eventually forming, and belonging to, a single entity.